1. Field of the Invention
The invention relates to a decorative sheet comprising a compression-molded core having one or more core layers, said core layers being prefabricated layers made from wood fibers and/or of cellulose fibers, surrounded and bound by hot-curing resin, and also to the use of a sheet of this type, and to processes for the production of the sheets.
2. Description of the Related Art
Sheets made from wood chips and wood fibers with a matrix of amino plastic or of phenolic resins or cement are known from the prior art, as are plywood sheets, these having uniform density and being suitable for outdoor use or use in damp indoor areas. However, unlike laminate sheets, sheets of this type are not maintenance-free, and their marked absorption of water through the unsealed surface means that they regularly require comprehensive treatment. Absorption of water can cause severe swelling with an increase in the thickness of the sheet. These sheets then have low strength and are unsightly.
U.S. Pat. No. 3,673,020 (which corresponds to German laid-open patent application 19 12 300) describes a decorative molded sheet made from sawdust and comprising a matrix of phenolic resin, the resin content being from 5 to 15% by weight, based on the dry weight of the sawdust. The weathering resistance of this molding is unsatisfactory. In weathering tests there is marked absorption of water with associated edge swelling and splitting of the compression-molded core, causing surface cracking after just a few weeks.
Lightweight structural panels as described in British Patent 1 015 803 comprise surface layers composed, for example, of wood shavings of minimum length about 50 mm. These wood shavings predominate within the total weight of the sheet, while the proportion of resin is only small, for example in the range from 6 to 10% by weight. The resin serves merely to bond the wood particles to one another. The density of these lightweight structural panels is relatively low, not more than 450 kg/m3.
U.S. Pat. No. 4,503,115 (which corresponds to European Patent 0 081 147) discloses a decorative structural panel composed of a compression-molded core made from wood particles surrounded by hot-curing phenol-formaldehyde resin. One or both surfaces of the core of the structural panel has/have decorative layers composed of a woven or nonwoven or other fabric or of a plastic film or a paper foil or wood veneer and/or of a layer formed by liquid coating. The wood fibers and/or cellulose fibers of the core have a maximum length of 20 mm, and the wood fibers here have been coated with the hot-curing phenol-formaldehyde resin in aqueous solution or dispersion. The amount of resin is more than 150 g for each 1000 g of dry fibers, and extends up to 900 g. The water content of the wood fibers is of the order of from 3 to 10% by weight, and the density of the structural panel is from 1100 to 1500 kg/m3. The decorative layer is composed in particular of a melamine-formaldehyde-resin-saturated decorative foil with a weight per unit area of from 80 to 400 g/m3. The decorative layer comprises, where appropriate, a filler and/or a dye. If the surface of the compression-molded core has been decorated by liquid coating using a crosslinked acrylic resin, urethane resin, epoxy resin or melamine resin, this liquid coating comprises, where appropriate, a filler and/or a dye.
This decorative structural panel is produced by arranging one or more mechanically precompacted layers based on wood fibers on top of one another and press-molding these with heat to form the core of the structural panel, with curing of the hot-curing phenol-formaldehyde resin of the precompacted layers. One or both surfaces of the core is/are provided with a decorative layer, which is either applied to the precompacted mat made from one or more layers, or applied to the core once it has been obtained by press-molding this mat.
This known structural panel has very consistent longitudinal and transverse dimensional stability, and even when subjected to the effects of severe changes in weather its dimensional stability remains at a high level. Compared with other known sheets made from wood chips, from wood fibers or from cellulose fibers, this structural panel has higher flexural strength, higher tensile strength and higher transverse strength, higher dimensional stability and reduced water absorption. This structural panel is a laminate made from previously press-molded wood fiber/resin layers and decorative layers, and its visible color remains unchanged over long periods since the weathering-resistant decorative layers protect the surfaces of the core.
In contrast, the object of the invention is to propose a decorative sheet of the type mentioned at the outset, and a process for producing the sheet, the sheet having the appearance of a sheet of solid wood with a patina effect on the surfaces of the sheets and with a coloration when wet which differs from that when the sheet is dry. The object includes retaining the transverse and longitudinal dimensional stability, and flexural strength and tensile strength, and also low water absorption, of the structural panel of U.S. Pat. No. 4,503,115 (equivalent to European Patent 0 081 147).
The object is achieved by means of the sheet described at the outset with a core layer which comprises wood fibers and/or cellulose fibers of length from 0.3 to 30 mm, and with some free fibers at the surface of the core layer. In one embodiment of the invention, the core layer comprises pigments, these having been selected from the group consisting of inorganic, colored oxidic pigments, inorganic pigments with hydroxyl groups, inorganic sulfidic pigments, carbon black pigments, phthalocyanine pigments and quinacridone pigments. The pigments have in particular been selected from the group consisting of titanium dioxide, zinc sulfide, zinc sulfate, red and yellow iron oxide, blue phthalocyanine pigments, lamp black pigments, oxidic black pigments and quinacridone pigments with violet-red hues.
It is useful that the weathering of the wood fibers and/or cellulose fibers on the surfaces of the core layer gives a gray-silver-colored patina. The gray-silver patina here overlies the original color of the core layer at the surfaces of the sheet.
In a process of the invention for producing sheets, one or more mechanically precompacted layers made from cellulose fibers and/or from wood fibers, enclosed by a hot-curing resin, are placed one on top of another layer and press-molded to one another, applying heat to the layers to bind them together to a core, curing the resin within the cure, and introducing pigments into the resin prior to resin-coating the cellulose fibers and/or wood fibers within the mechanically precompacted layers. According to the process, without applying any liquid coating to the surfaces or applying a coating of decorative films, plastic films, paper foils or wood veneers, the precompacted layers are press-molded to one another at a temperature of from 120 to 180xc2x0 C. and at a pressure of from 30 to 100 bar to form sheets.
In using the sheets of the invention as a facade cladding in the form of a curtain facade, of a panel screening a wall or roof, or of cladding for a balcony, or of a parapet panel or apron panel, the sheets are subjected to direct external weathering which causes slight weathering changes in the resin and the wood fibers and/or cellulose fibers which are directly arranged at the surfaces of the sheets, changing the original color of the sheets. Specifically, the slight weathering changes in the resin release some of the fibers at the surfaces of the sheets and cause due to this release some slight color changes to the sheets, but not any noticeable change in the physical, mechanical or chemical properties of the sheets. During this process, the surfaces of the sheets are given a patina effect, caused by a translucent gray-silver color which overlays the surfaces. When dry, the sheets then have the appearance of sheets of solid natural wood with a patina effect. If the sheets becomes moist or wet as a result of weathering, this patina effect disappears and the sheets is given their original color. The color change on the transition from the dry state to the wet state of a sheet is explained by the different reflection properties of the sheet in these states. In the dry state, the irregular surface with its slight weathering changes reflects the light nonuniformly and nondirectionally. In the wet state, the surface of the sheet has been moistened and becomes uniform, reflecting the light more directionally and producing a perceived color similar to that of the sheet prior to its slight change by weathering. As a result of these color changes, the sheet gives the impression that it is natural wood and certainly not a laminate made from a variety of layers press-molded to one another. It has all of the technical and physical advantages of a laminated sheet composed of layers and press-molded at high pressure and temperature, for example high strength, low tendency to split on impact, dimensional stability and low moisture absorption. Various types of natural wood can be imitated by appropriately selecting and combining or mixing the pigments added.
The patina effect on the sheet may in particular be utilized for architectural purposes, since the color of facade cladding made from sheets of this type appears different in different types of weather, producing the impression that the facade cladding has been made from solid natural timber. The patina effect normally occurs with all types of wood, and architects give preference to the use of some types of wood as decorative elements due to their particularly beautiful patina effect. Teak and jacaranda are among these types of wood.
U.S. Pat. No. 4,503,115 gives details of the production of the wood fibers for the layers which are press-molded to form the core, and the disclosure content of that publication is incorporated into the present description by way of reference.